The present invention relates to method and apparatus for recording information onto an optical disk. More specifically, the invention relates to a method for recording, on individual optical disks, individual information different from disk to disk.
A read only optical disk such as a DVD (Digital Video Disk) has a synthetic resin substrate on which a pattern of pits and projections, or a so-called pit pattern, corresponding to recorded information is formed, and a recording layer which is formed of a metallic thin layer of gold, aluminum or the like so as to cover the pit pattern of the substrate. The pattern of pits and projections of the recording layer is read by a laser beam, by which information such as image, audio, and character data is obtained.
For the formation of the pit pattern on the synthetic resin substrate, a pit pattern is transferred and formed onto the synthetic resin substrate from a metal master disk or the like on which the pit pattern has previously been formed. Therefore, from one master disk, optical disks on which exactly the same information is recorded are produced efficiently. Generally, read only optical disks, once produced, do not allow any additional information to be written.
Meanwhile, in terms of management for production and storage or distribution and marketing of optical disks, there are some cases where serial numbers or identification codes are attached to individual optical disks.
Thus, there has been proposed a technique that individual information differing from optical disk to optical disk, such as serial numbers or identification codes, is additionally written by writing bar codes on produced optical disks with a laser beam. In the Japanese Patent Application No. 8-8910 that the present inventor has previously filed for patent, is disclosed a technique for reading individual information of a bar code through processes of applying a laser beam to the recording layer inside the optical disk to melt and remove the metallic thin film that forms the recording layer into the bar code pattern, and reading differences in reflection characteristics between the removed portion and its surrounding recording layer by a laser beam different from the above laser beam for writing.
With the adoption of such a method for additionally writing information with bar codes, any arbitrary information can be additionally written with ease afterwards onto optical disks produced by the same manufacturing process as the conventional. The added information can be read easily only by adding a simple circuit to an ordinary player. As a result, it becomes possible to implement customer management, illegal copy prevention and the like securely and efficiently by utilizing the serial numbers or identification codes. Moreover, because once written bar codes cannot be rewritten afterwards, information alteration can be prevented.
However, with the above-described method of additionally writing bar codes, there may occur optical disks from which bar codes can hardly be read or optical disks for which erroneous information is read, that is, defective products. Since a bar code comprising an array of minute linear patterns is written by melting with a laser beam the metallic thin film that forms the recording layer within the optical disk, there arise some cases where the bar code is party missing or adjacent bar codes join together so that bar codes of correct shape are not made.
Defective products of optical disks that are generated in the process of additionally writing the bar codes are usually discarded. In such a case, individual information written in the defective optical disks is also discarded. For example, in the case where serial numbers are additionally written in optical disks, if optical disks having particular serial numbers are discarded, the serial numbers would be unused numbers. In product management using serial numbers, the presence of unused numbers somewhere in a the sequence would cause great trouble in the management. Without continuous management of information regarding about unused numbers, it would be difficult to determine whether later occurrence of an unused number in the serial numbers is due to the marketing and distribution or the manufacturing process. Also, a need of an optical disk having a particular identification code, if the particular identification code is absent, the need could not be fulfilled.